Light-emitting element having electron-trapping layer

ABSTRACT

A light-emitting element is provided, including a first electrode and a second electrode, a first layer including first and second organic compounds, the first layer being formed between the first electrode and the second electrode wherein the first organic compound is capable of emitting a first light and the second organic compound has an electron transporting property, and a second layer including third and fourth organic compounds, the second layer being formed between the first layer and the second electrode wherein the third organic compound is capable of emitting a second light and has an electron trap property and the fourth organic compound has an electron transporting property.

This application is a continuation of U.S. application Ser. No.11/804,747 filed on May 18, 2007 now U.S. Pat. No. 8,030,646.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting element of a currentexcitation type. Moreover, the present invention relates to alight-emitting device and an electronic device each having thelight-emitting element. In more detail, the present invention relates toa long-life light-emitting device which is superior in color purity.Further, the present invention relates to a long-life light-emittingdevice and electronic device which are superior in color purity.

2. Description of the Related Art

In recent years, research and development have been extensivelyconducted on light-emitting elements using electroluminescence. As abasic structure of these light-emitting elements, a layer containing asubstance with a light-emitting property is interposed between a pair ofelectrodes. By application of voltage to this element, light emissionfrom a substance with a light-emitting property can be obtained.

Since such a light-emitting element is a self-luminous type, there areadvantages such as higher visibility of a pixel than visibility of aliquid crystal display, and unnecessity of a backlight. Accordingly,such a light-emitting element is considered to be suitable as a flatpanel display element. In addition, such a light-emitting element can bemanufactured to be thin and light, which is a great advantage. Moreover,the light-emitting element has a feature that response speed isextremely fast.

Furthermore, since such a light-emitting element can be formed into afilm form, planar light emission can be easily obtained by formation ofa large-area element. This characteristic is difficult to be obtained bya point light source typified by an incandescent lamp or an LED, or aline light source typified by a fluorescent lamp. Therefore, thelight-emitting element has a high utility value as a plane light sourcethat can be applied to lighting or the like.

Although the light-emitting elements using electroluminescence areclassified roughly in accordance with whether they use an organiccompound or an inorganic compound as a substance having a light-emittingproperty, in the present invention, an organic compound is used for thesubstance with a light-emitting property.

In that case, by application of voltage to the light-emitting element,electrons and holes are injected from the pair of electrodes into thelayer containing an organic compound with a light-emitting property tocause current flow. Then, by recombination of these carriers (electronsand holes), the organic compound with a light-emitting property forms anexcited state, and light is emitted when the excited state returns to aground state. Because of such a mechanism, this kind of light-emittingelement is referred to as a light-emitting element of a currentexcitation type.

It is to be noted that an excited state formed by an organic compoundcan be a singlet excited state or a triplet excited state. Lightemission from the singlet excited state is referred to as fluorescence,and light emission from the triplet excited state is referred to asphosphorescence.

In order to overcome many problems derived from materials of such alight-emitting element and to improve its element characteristics,improvement of an element structure, material development, and so on arecarried out.

For example, Patent Document 1 (Patent Document 1: Japanese PublishedPatent Application No. 2000-068057) discloses a long-life organic ELelement in which two light-emitting layers are included to be doped withplural kinds of fluorescent substances having different colors, wherebycolor stability is secured. However, in Patent Document 1, since dopingwith plural kinds of fluorescent substances having different colors isperformed, light emission with good color purity cannot be obtained,even though white light emission with good color stability can beobtained.

Further, a light-emitting element using an organic compound with alight-emitting property can be driven at lower voltage compared with alight-emitting element using an inorganic compound with a light-emittingproperty. However, there is a problem that the element has the shortlife. Accordingly, the light-emitting element is desired to have thelonger life.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide along-life light-emitting element which is superior in color purity, in acase where an organic compound is used for a substance with alight-emitting property. It is another object to provide a long-lifelight-emitting device and electronic device which are superior in colorpurity.

As a result of diligent study, the present inventors have found out thatthe objects can be solved by providing a light-emitting region in thevicinity of the center of the light-emitting layer. That is, the presentinventors have found out that the objects can be solved by providing alight-emitting region not at an interface between a light-emitting layerand a hole transporting layer or an interface between a light-emittinglayer and an electron transporting layer but in the vicinity of thecenter of the light-emitting layer in a case where the hole transportinglayer and the electron transporting layer exist between both electrodes.Further, the present inventors have found out that the objects can besolved by providing a light-emitting region not at an interface betweena light-emitting layer and a hole transporting layer or an interfacebetween a light-emitting layer and an electron transporting layer but inthe vicinity of the center of the light-emitting layer also in a casewhere the hole transporting layer and the electron transporting layer donot exist. The present invention is developed based on expertisedescribed above, and a light-emitting element of the present inventionhas many modes.

According to a first mode, a light-emitting layer is included between afirst electrode and a second electrode, where the light-emitting layerhas a first layer and a second layer, the first layer has a firstorganic compound and a second organic compound, the second layer has athird organic compound and a fourth organic compound, the first layer isprovided on a first electrode side of the second layer, the secondorganic compound has an electron transporting property, the thirdorganic compound has an electron trap property, the fourth organiccompound has an electron transporting property, emission color of thefirst organic compound and emission color of the third organic compoundare similar colors, and light emission from the first organic compoundis obtained by application of voltage to the first electrode and thesecond electrode so that a potential of the first electrode is higherthan a potential of the second electrode.

According to a second mode, a light-emitting layer is included between afirst electrode and a second electrode, where the light-emitting layerhas a first layer and a second layer, the first layer has a firstorganic compound and a second organic compound, the second layer has athird organic compound and a fourth organic compound, the first layer isprovided on a first electrode side of the second layer, the secondorganic compound has an electron transporting property, the thirdorganic compound has a lowest unoccupied molecular orbital level lowerthan a lowest unoccupied molecular orbital level of the fourth organiccompound by 0.3 eV or more, the fourth organic compound has an electrontransporting property, emission color of the first organic compound andemission color of the third organic compound are similar colors, andlight emission from the first organic compound is obtained byapplication of voltage to the first electrode and the second electrodeso that a potential of the first electrode is higher than a potential ofthe second electrode.

According to a third mode, a light-emitting layer is included between afirst electrode and a second electrode, where the light-emitting layerhas a first layer and a second layer, the first layer has a firstorganic compound and a second organic compound, the second layer has athird organic compound and a fourth organic compound, the first layer isprovided on a first electrode side of the second layer, the secondorganic compound has an electron transporting property, the thirdorganic compound has a lowest unoccupied molecular orbital level lowerthan a lowest unoccupied molecular orbital level of the fourth organiccompound by 0.3 eV or more, the fourth organic compound has an electrontransporting property, a difference between a peak value of an emissionspectrum of the first organic compound and a peak value of an emissionspectrum of the third organic compound is 0 nm or more and less than 30nm, and light emission from the first organic compound is obtained byapplication of voltage to the first electrode and the second electrodeso that a potential of the first electrode is higher than a potential ofthe second electrode.

According to a fourth mode, an electron transporting layer and a holetransporting layer are included between a first electrode and a secondelectrode, where a first layer and a second layer are included betweenthe electron transporting layer and the hole transporting layer, thefirst layer has a first organic compound and a second organic compound,the second layer has a third organic compound and a fourth organiccompound, the first layer is provided on a first electrode side of thesecond layer, the second organic compound has an electron transportingproperty, the third organic compound has an electron trap property, thefourth organic compound has an electron transporting property, emissioncolor of the first organic compound and emission color of the thirdorganic compound are similar colors, and light emission from the firstorganic compound is obtained by application of voltage to the firstelectrode and the second electrode so that a potential of the firstelectrode is higher than a potential of the second electrode.

According to a fifth mode, an electron transporting layer and a holetransporting layer are included between a first electrode and a secondelectrode, where a first layer and a second layer are included betweenthe electron transporting layer and the hole transporting layer, thefirst layer has a first organic compound and a second organic compound,the second layer has a third organic compound and a fourth organiccompound, the first layer is provided on a first electrode side of thesecond layer, the second organic compound has an electron transportingproperty, the third organic compound has a lowest unoccupied molecularorbital level lower than a lowest unoccupied molecular orbital level ofthe fourth organic compound by 0.3 eV or more, the fourth organiccompound has an electron transporting property, emission color of thefirst organic compound and emission color of the third organic compoundare similar colors, and light emission from the first organic compoundis obtained by application of voltage to the first electrode and thesecond electrode so that a potential of the first electrode is higherthan a potential of the second electrode.

According to a sixth mode, an electron transporting layer and a holetransporting layer are included between a first electrode and a secondelectrode, where a first layer and a second layer are included betweenthe electron transporting layer and the hole transporting layer, thefirst layer has a first organic compound and a second organic compound,the second layer has a third organic compound and a fourth organiccompound, the first layer is provided on a first electrode side of thesecond layer, the second organic compound has an electron transportingproperty, the third organic compound has a lowest unoccupied molecularorbital level lower than a lowest unoccupied molecular orbital level ofthe fourth organic compound by 0.3 eV or more, the fourth organiccompound has an electron transporting property, a difference between apeak value of an emission spectrum of the first organic compound and apeak value of an emission spectrum of the third organic compound is 0 nmor more and less than 30 nm, and light emission from the first organiccompound is obtained by application of voltage to the first electrodeand the second electrode so that a potential of the first electrode ishigher than a potential of the second electrode.

In these many modes of the light-emitting element, the first layer andthe second layer are preferably provided to be in contact with eachother.

It is to be noted that the present invention includes in its category alight-emitting device having the aforementioned light-emitting element.The light-emitting device in this specification includes an imagedisplay device, a light-emitting device, a light source (including alighting device), or the like. Further, the light-emitting device alsoincludes a module in which a connector such as an FPC (Flexible PrintedCircuit), a TAB (Tape Automated Bonding) tape, or a TCP (Tape CarrierPackage) is attached to a panel on which the light-emitting element isformed. The light-emitting device in this specification also includes amodule in which a printed wiring board is provided at an end of a TABtape or a TCP, and also includes a module or the like in which an IC(Integrated Circuit) is directly mounted on the light-emitting elementby a COG (Chip On Glass) method. Further, the light-emitting device inthe present invention also includes a case where a light-emittingelement includes a control means for controlling light emission of thelight-emitting element.

The present invention includes in its category an electronic device aswell, in which the light-emitting element of the present invention isused for a display device. Further, another aspect of the presentinvention is an electronic device including a display device, where thedisplay device is provided with the light-emitting element describedabove and a control means for controlling light emission of thelight-emitting element.

In the light-emitting element of the present invention, a light-emittingregion is formed not at an interface between a light-emitting layer anda hole transporting layer or an interface between a light-emitting layerand an electron transporting layer but in the vicinity of the center ofthe light-emitting layer. Further, a long-life light-emitting element inwhich an element is not easily deteriorated can be obtained. Also in acase where a hole transporting layer and an electron transporting layerdo not exist, the similar effect can be obtained by providing alight-emitting region not at an interface between both electrodes and alight-emitting layer but in the vicinity of the center of thelight-emitting layer.

Further, in the light-emitting element of the present invention,emission color of the first organic compound and emission color of thethird organic compound are similar colors. Accordingly, light emissionwith good color purity can be obtained even when not only the firstorganic compound but also the third organic compound emits light.

Furthermore, by application of the light-emitting element of the presentinvention to a light-emitting device and an electronic device, along-life light-emitting device and electronic device which are superiorin color purity can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are views illustrating a light-emitting element of thepresent invention;

FIG. 2 is a view illustrating a light-emitting element of the presentinvention;

FIG. 3 is a view illustrating a light-emitting element of the presentinvention;

FIGS. 4A and 4B are a top view and a cross-sectional view, respectively,of a light-emitting device of the present invention;

FIG. 5 is a view illustrating a light-emitting device of the presentinvention;

FIGS. 6A to 6D are views illustrating electronic devices of the presentinvention;

FIG. 7 is a view illustrating an electronic device of the presentinvention;

FIG. 8 is a view illustrating a lighting device of the presentinvention;

FIG. 9 is a view illustrating a lighting device of the presentinvention;

FIG. 10 is a view illustrating a light-emitting element of embodiments;

FIG. 11 is a graph showing a current density-luminance characteristic ofa light-emitting element manufactured in Embodiment 1;

FIG. 12 is a graph showing a voltage-luminance characteristic of alight-emitting element manufactured in Embodiment 1;

FIG. 13 is a graph showing a luminance-current efficiency characteristicof a light-emitting element manufactured in Embodiment 1;

FIG. 14 is a graph showing an emission spectrum of a light-emittingelement manufactured in Embodiment 1;

FIG. 15 is a graph showing a current density-luminance characteristic ofa light-emitting element manufactured in Embodiment 2;

FIG. 16 is a graph showing a voltage-luminance characteristic of alight-emitting element manufactured in Embodiment 2;

FIG. 17 is a graph showing a luminance-current efficiency characteristicof a light-emitting element manufactured in Embodiment 2;

FIG. 18 is a graph showing an emission spectrum of a light-emittingelement manufactured in Embodiment 2;

FIG. 19 is a graph showing a reduction reaction characteristic of Alq;

FIG. 20 is a graph showing a reduction reaction characteristic of DPQd;and

FIG. 21 is a view illustrating a light-emitting element of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiment modes of the present invention will be describedin detail with reference to drawings. However, the present invention isnot limited to explanation to be given below, and it is to be easilyunderstood that modes and details thereof can be variously modifiedwithout departing from the purpose and the scope of the presentinvention. Therefore, the present invention should not be interpreted asbeing limited to the description of the embodiment modes to be givenbelow.

It is to be noted that, in the present specification, “composition”refers to not only a state where a plurality of materials are simplymixed but also a state where charges are given and received betweenmaterials by the mixture.

Embodiment Mode 1

One mode of a light-emitting element of the present invention will bedescribed with reference to FIG. 1A.

A light-emitting element of the present invention has a plurality oflayers between a pair of electrodes. The plurality of layers are acombination of layers formed of a material with a high carrier injectingproperty and a material with a high carrier transporting property whichare stacked so that a light-emitting region is formed in a region awayfrom the electrodes, that is, recombination of carriers is performed inan area away from the electrodes.

In this embodiment mode, a light-emitting element includes a firstelectrode 102, a second electrode 104, and an EL layer 103 providedbetween the first electrode 102 and the second electrode 104. It is tobe noted that the description will be made below regarding the firstelectrode 102 as an anode and the second electrode 104 as a cathode. Inother words, the description will be made below regarding light emissionas being obtained when voltage is applied to the first electrode 102 andthe second electrode 104 so that a potential of the first electrode 102is higher than that of the second electrode 104.

A substrate 101 is used as a base of the light-emitting element. As thesubstrate 101, glass, plastic, or the like may be used, for example.Other materials than those may be used as long as the materials functionas a base in the process of manufacturing the light-emitting element.

As a material used for the first electrode 102, a metal, an alloy, anelectroconductive compound, a mixture thereof, or the like with a highwork function (specifically, a work function of 4.0 eV or higher ispreferable) is preferably used. Specifically, for example, indiumoxide-tin oxide (ITO: Indium Tin Oxide), indium oxide-tin oxidecontaining silicon or silicon oxide, indium oxide-zinc oxide (IZO:Indium Zinc Oxide), indium oxide containing tungsten oxide and zincoxide (IWZO), and the like are given. Such conductive metal oxide filmsare usually deposited by sputtering, but may also be formed by using asol-gel method or the like. For example, indium oxide-zinc oxide (TZO)can be formed by a sputtering method using a target in which zinc oxideof 1 to 20 wt % is added to indium oxide. Indium oxide containingtungsten oxide and zinc oxide (IWZO) can be formed by a sputteringmethod using a target containing tungsten oxide of 0.5 to 5 wt % andzinc oxide of 0.1 to 1 wt % with respect to indium oxide. Other thanthese, gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium(Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium(Pd), a nitride of a metal material (such as titanium nitride), and thelike are given.

The stacked structure of layers in the EL layer 103 is not limited inparticular, and may include a layer formed of a substance with a highelectron transporting property, a substance with a high holetransporting property, a substance with a high electron injectingproperty, a substance with a high hole injecting property, a substancewith a bipolar property (a substance with a high electron and holetransporting property), or the like and a light-emitting layer to beshown in this embodiment mode to be appropriately combined. For example,the EL layer 103 can be formed by an appropriate combination of a holeinjecting layer, a hole transporting layer, a light-emitting layer, anelectron transporting layer, an electron injecting layer, and the like.Hereinafter, materials for forming each layer will be specificallydescribed below. As one example, FIGS. 1A to 1C show a structure inwhich a first electrode 102, a hole transporting layer 112, alight-emitting layer 111, an electron transporting layer 113, and asecond electrode 104 are sequentially stacked.

A hole injecting layer may be provided between the first electrode 102and the hole transporting layer 112. A hole injecting layer is a layerthat contains a substance with a high hole injecting property. As thesubstance with a high hole injecting property, molybdenum oxide,vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or thelike can be used. In addition, it is possible to use aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) or copper phthalocyanine (abbreviation: CuPc), a high moleculesuch as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid)(PEDOT/PSS), or the like to form the hole injecting layer.

Alternatively, as the hole injecting layer, a composite material of asubstance with a high hole transporting property containing an acceptorsubstance may be used. It is to be noted that, by using the substancewith a high hole transporting property containing an acceptor substance,a material used to form an electrode may be selected regardless of itswork function. In other words, besides a material with a high workfunction, a material with a low work function may also be used as thefirst electrode 102. As the acceptor substance,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, atransition metal oxide can be given. In addition, an oxide of metalsthat belong to Group 4 to Group 8 of the periodic table can be given.Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromiumoxide, molybdenum oxide, tungsten oxide, manganese oxide, and rheniumoxide are preferable since their electron accepting property is high.Among these, molybdenum oxide is especially preferable since it isstable in the air and its hygroscopic property is low so that it can beeasily treated.

As a substance with a high transporting property used for the compositematerial, various compounds such as an aromatic amine compound,carbazole derivatives, aromatic hydrocarbon, and a high molecularcompound (such as oligomer, dendrimer, or polymer) can be used.Specifically, a substance having hole mobility of 10⁻⁶ cm²/Vs or higheris preferably used. However, other substances than those may also beused as long as the substances have hole transporting properties higherthan electron transporting properties. The organic compounds with highhole transporting properties which can be used for the compositematerial will be specifically shown below.

For example, the followings can be given as the aromatic amine compound:N,N′-bis(4-methylphenyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA); 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB);4,4′-bis(N-{4-[N-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD);1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B); and the like.

As the carbazole derivatives that can be used for the compositematerial, the followings can be given specifically:3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1);3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2);3-[N-(1-naphtyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1); and the like.

Moreover, as the other carbazole derivatives that can be used for thecomposite material, 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP);1,3,5-tris[4-(N-carb azolyl)phenyl]benzene (abbreviation: TCPB);9-[4-(N-carbazolyl)]phenyl-10-phenylanthracene (abbreviation: CzPA);1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene; or the likecan also be used.

As the aromatic hydrocarbon that can be used for the composite material,the followings can be given for example:2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA);2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA);2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA);9,10-di(2-naphthyl)anthracene (abbreviation: DNA);9,10-diphenylanthracene (abbreviation: DPAnth); 2-tert-butylanthracene(abbreviation: t-BuAnth); 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA);9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butyl-anthracene;9,10-bis[2-(1-naphthyl)phenyl]anthracene;2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene;2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene; 9,9′-bianthryl;10,10′-diphenyl-9,9′-bianthryl;10,10′-bis(2-phenylphenyl)-9,9′-bianthryl;10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl; anthracene;tetracene; rubrene; perylene; 2,5,8,11-tetra(tert-butyl)perylene; andthe like. Besides those, pentacene, coronene, or the like can also beused. As described above, the aromatic hydrocarbon which has holemobility of 1×10⁻⁶ cm²/Vs or higher and which has 14 to 42 carbon atomsis particularly preferable.

The aromatic hydrocarbon that can be used for the composite material mayhave a vinyl skeleton. As such an aromatic hydrocarbon, the followingsare given for example: 4,4′-bis(2,2-diphenylvinyl)biphenyl(abbreviation: DPVBi); 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA); and the like.

Moreover, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK) or poly(4-vinyltriphenylamine) (abbreviation: PVTPA)can also be used.

The hole transporting layer 112 is a layer that contains a substancewith a high hole transporting property. As the substance with a highhole transporting property, for example, an aromatic amine compound suchas 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), or4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB) can be used. These substances mainly are substanceseach having hole mobility of 10⁻⁶ cm²/Vs or higher. However, othersubstances than these may also be used as long as the hole transportingproperties thereof are higher than the electron transporting properties.The layer containing a substance with a high hole transporting propertyis not limited to a single layer, and two or more layers containing theaforementioned substances may be stacked.

The light-emitting layer 111 is a layer containing a substance with ahigh light-emitting property. In the light-emitting element of thepresent invention, the light-emitting layer has a first layer 121 and asecond layer 122. The first layer 121 has a first organic compound and asecond organic compound, and the second layer 122 has a third organiccompound and a fourth organic compound.

The first organic compound included in the first layer 121 is asubstance with a high light-emitting property, and various materials canbe used. Specifically, as a light-emitting material exhibiting emissionof bluish light,N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstylbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA), and the like are given. As a light-emittingmaterial exhibiting emission of greenish light,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazole-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazole-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracene-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracene-9-amine(abbreviation: DPhAPhA), and the like are given. As a light-emittingmaterial emission of yellowish light, rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),and the like are given. As a light-emitting material exhibiting emissionof reddish light,N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,13-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD), and the like are given.

The second organic compound included in the first layer 121 is asubstance with a higher electron transporting property than a holetransporting property and a substance by which the substance with a highlight-emitting property described above is dispersed. Preferably, thesecond organic compound is a so-called bipolar material in which themobility of holes or electrons is 10 times or less than the mobility ofthe other. Specifically, 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA), tris(8-quinolinolato)aluminum(III) (abbreviation:Alq), 4,4′-(quinoxaline-2,3-diyl)bis(N,N-diphenylaniline) (abbreviation:TPAQn), 9,10-diphenylanthracene (abbreviation: DPAnth),N,N′-(quinoxaline-2,3-diyldi-4,1-phenylene)bis(N-phenyl-1,1′-biphenyl-4-amine)(abbreviation: BPAPQ),4,4′-(quinoxaline-2,3-diyl)bis{N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylaniline}(abbreviation: YGAPQ), 9,10-diphenylanthracene (abbreviation: DPAnth),and the like are given.

The third organic compound contained in the second layer 122 is anorganic compound having a function of trapping electrons. Accordingly,the third organic compound is preferably an organic compound having alowest unoccupied molecular orbital level (LUMO level) lower than alowest unoccupied molecular orbital level (LUMO level) of the fourthorganic compound contained in the second layer 122 by 0.3 eV or more.The third organic compound may emit light; however, in that case,emission color of the first organic compound and emission color of thethird organic compound are preferably similar colors. That is, forexample, in a case where the first organic compound exhibits emission ofbluish light like YGA2S or YGAPA, the third organic compound ispreferably a substance exhibiting emission of blue to blue green lightsuch as acridone, coumarin 102, coumarin 6H, coumarin 480D, or coumarin30. In a case where the first organic compound exhibits emission ofgreenish light like 2PCAPA, 2PCABPhA, 2DPAPA, 2DPABPhA, 2YGABPhA, orDPhAPhA, the third organic compound is preferably a substance exhibitingemission of blue green to yellowish green light such asN,N′-dimethylquinacridone (abbreviation: DMQd),N,N′-diphenylquinacridone (abbreviation: DPQd),9,18-dihydrobenzo[h]benzo[7,8]quino[2,3-b]acridine-7,16-dione(abbreviation: DMNQd-1),9,18-dimethyl-9,18-dihydrobenzo[h]benzo[7,8]quino[2,3-b]acridine-7,16-dione(abbreviation: DMNQd-2), coumarin 30, coumarin 6, coumarin 545T,coumarin 153, or the like. In a case where the first organic compoundexhibits emission of yellowish light like rubrene or BPT, the thirdorganic compound is preferably a substance exhibiting emission ofyellowish green to yellowish orange light such as DMQd or(2-{2-[4-(9H-carbazol-9-yl)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCMCz). In a case where the first organic compoundexhibits emission of reddish light like p-mPhTD or p-mPhAFD, the thirdorganic compound is preferably a substance exhibiting emission of orangeto red light such as(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),{2-(1,1-dimethylethyl)-6-[2-(2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB), or Nile red. The above-described compounds arecompounds having lower LUMO level among compounds used for alight-emitting element and show a favorable electron trap property bybeing added to the fourth organic compound to be described below.

Though the description is made as above, as the third organic compound,quinacridone derivatives such as DMQd, DPQd, DMNQd-1, or DMNQd-2 arepreferable among the substances enumerated above because quinacridonederivatives are chemically stable. That is, when quinacridonederivatives are applied, the life of a light-emitting element can beespecially extended. Further, since quinacridone derivatives exhibitsemission of greenish light, the element structure of the light-emittingelement of the present invention is especially effective for alight-emitting element exhibiting emission of greenish light. Sincegreen is a color that needs highest luminance than other colors when afull-color display is manufactured, there is a case where the level ofdeterioration of green is higher than that of other colors. However, itcan be improved by the application of the present invention.

The fourth organic compound contained in the second layer 122 is anorganic compound with an electron transporting property, that is, asubstance with a higher electron transporting property than a holetransporting property. Specifically, tris(8-quinolinolato)aluminum(III)(abbreviation: Alq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis[2-(2-benzoxazolyl)phenolato]zinc(II)(abbreviation: ZnPBO), bis[2-(2-benzothiazolyl)phenolato]zinc(II)(abbreviation: ZnBTZ), and the like are given. Further, as describedabove, LUMO level of the third organic compound is preferably lower thanLUMO level of the fourth organic compound by 0.3 eV or more.Accordingly, the fourth organic compound may be appropriately selectedin accordance with a type of the third organic compound to be used sothat such a condition is satisfied. For example, as described below inEmbodiment, in a case where DPQd is used as the third organic compound,the above condition is satisfied when Alq is used as the fourth organiccompound.

Emission color of the first organic compound and emission color of thethird organic compound are preferably similar colors. Accordingly, adifference between a peak value of an emission spectrum of the firstorganic compound and a peak value of an emission spectrum of the thirdorganic compound is preferably within 30 nm. When the difference iswithin 30 nm, emission color of the first organic compound and emissioncolor of the third organic compound are similar colors. Accordingly,even in a case where the third organic compound emits light due tochange in voltage or the like, change in emission color of thelight-emitting element can be suppressed. However, the third organiccompound has no necessity to emit light. For example, in a case whereemission efficiency of the first organic compound is higher than that ofthe third organic compound, the concentration of the third organiccompound in the second layer 122 is preferably adjusted so that onlylight emitted from the first organic compound is substantially obtained(the concentration is slightly lowered so that light emission from thethird organic compound is suppressed). In this case, emission color ofthe first organic compound and emission color of the third organiccompound are similar colors (that is, the first organic compound and thethird organic compound have almost the same energy gap). Accordingly,energy transfer from the first organic compound to the third organiccompound does not easily occur, and thus high emission efficiency isobtained.

The electron transporting layer 113 is a layer that contains a substancewith a high electron transporting property. For example, the electrontransporting layer 113 is a layer containing a metal complex or the likehaving a quinoline skeleton or a benzoquinoline skeleton, such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq). Alternatively, a metal complex or the like havingan oxazole-based or thiazole-based ligand, such asbis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO) orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ) canalso be used. Further alternatively, other than the metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can also be used. Thesubstances mentioned here mainly are substances each having electronmobility of 10⁻⁶ cm²/Vs or higher. The electron transporting layer maybe formed of other substances than those described above as long as thesubstances have higher electron transporting properties than holetransporting properties. Furthermore, the electron transporting layer isnot limited to a single layer, and two or more layers made of theaforementioned substances may be stacked.

Further, an electron injecting layer that is a layer containing asubstance with a high electron injecting property may be providedbetween the electron transporting layer 113 and the second electrode104. As the electron injecting layer, a compound of an alkali metal oran alkaline earth metal such as lithium fluoride (LiF), cesium fluoride(CsF), or calcium fluoride (CaF₂) can be used. Further, a layer of asubstance with an electron transporting property containing an alkalimetal or an alkaline earth metal, such as Alq containing magnesium, maybe used. With the use of a layer of a substance with an electrontransporting property containing an alkali metal or an alkaline earthmetal as the electron injecting layer, electron injection from thesecond layer 104 is performed efficiently, which is preferable.

As a substance used for the second electrode 104, a metal, an alloy, anelectroconductive compound, or a mixture thereof, or the like with a lowwork function (specifically, a work function of 3.8 eV or lower ispreferable) can be used. As a specific example of such a cathodematerial, an element that belongs to Group 1 or 2 of the periodic table,that is, an alkali metal such as lithium (Li) or cesium (Cs), analkaline earth metal such as magnesium (Mg), calcium (Ca), or strontium(Sr), an alloy containing these (magnesium-silver, aluminum-lithium), arare earth metal such as europium (Eu) or ytterbium (Yb), an alloycontaining these, and the like are given. However, by the electroninjecting layer being provided between the second electrode 104 and theelectron transporting layer, various conductive materials such asaluminum, silver, ITO, or indium oxide-tin oxide containing silicon orsilicon oxide can be used as the second electrode 104 regardless oftheir work function.

Various methods can be used for forming the EL layer 103, regardless ofa dry method or a wet method. For example, a vacuum evaporation method,an ink-jet method, a spin coat method, or the like may be used.Furthermore, each electrode or each layer may be formed by a differentfilm deposition formation method.

The light-emitting element of the present invention that has thestructure as the above emits light when a current flows due to apotential difference generated between the first electrode 102 and thesecond electrode 104 and holes and electrons are recombined in the ELlayer 103. More specifically, the light-emitting element of the presentinvention has a structure in which, in the light-emitting layer 111 inthe EL layer 103, a light-emitting region is formed in the first layer121 and in the vicinity of an interface between the first layer 121 andthe second layer 122. This principle will be described below.

FIG. 21 is an example of a band diagram of the light-emitting element ofthe present invention shown in FIGS. 1A to 1C. In FIG. 21, holesinjected from the first electrode 102 are injected into the first layer121 through the hole transporting layer 112. Here, the second organiccompound included in the first layer 121 is a substance with a higherelectron transporting property than an electron transporting property,and preferably, a so-called bipoplar material having a difference inmobility of holes and electrons within 10 times; therefore, holesinjected into the first layer 121 is slowly transferred. Accordingly, ifa conventional light-emitting element provided with no second layer 122is used, a light-emitting region is formed in the vicinity of aninterface between the hole transporting layer 112 and the first layer121. In that case, there is a fear that electrons reach the holetransporting layer 112 and deteriorate the hole transporting layer 112.Further, if the amount of electrons reaching the hole transporting layer112 is increased with time, recombination probability is lowered withtime. As a result, luminance degradation with time is caused, leading tothe short life of the element.

The light-emitting element of the present invention has a feature thatthe second layer 122 is further provided in the light-emitting layer111. Electrons injected from the second electrode 104 are injected intothe second layer 122 through the electron transporting layer 113. Here,the second layer 122 has a structure in which the third organic compoundhaving a function of trapping electrons is added to the fourth organiccompound with an electron transporting property. Therefore, electronsinjected into the second layer 122 are slowly transferred, and thuselectron injection into the first layer 121 is controlled. As a result,in the light-emitting element of the present invention, a light-emittingregion that is conventionally formed in the vicinity of the interfacebetween the hole transporting layer 112 and the first layer 121 isformed in the first layer 121 and in the vicinity of the interfacebetween the first layer 121 and the second layer 122. Therefore, apossibility that electrons reach the hole transporting layer 112 anddeteriorate the hole transporting layer 112 becomes small. Similarly, apossibility that holes reach the electron transporting layer 113 anddeteriorate the electron transporting layer 113 is small because thesecond organic compound in the first layer 121 has an electrontransporting property.

Further, in the present invention, it is important that, in the secondlayer 122, an organic compound having a function of trapping electronsis added to an organic compound with an electron transporting property,rather than simply applying a substance having slow electron mobility asthe second layer 122. With such a structure, it is possible not only tocontrol electron injection into the first layer 121, but also to preventthe amount of the controlled electron injection from being changed withtime. Further, the amount of holes in the first layer 121 is not easilychanged with time as well because the second organic compound in thefirst layer 121 has an electron transporting property and the firstorganic compound that is a light-emitting substance is added to thefirst layer 121. From the above, in the light-emitting element of thepresent invention, a phenomenon that recombination probability islowered because carrier balance becomes worse with time in thelight-emitting element can be prevented. Accordingly, luminancedegradation with time can be suppressed, leading to improvement of thelife of the element.

In the above explanation, taking as an example a case where the holetransporting layer 112 and the electron transporting layer 113 exist,the following description is made: a phenomenon that recombinationprobability is lowered because carrier balance becomes worse with timein the light-emitting element can be prevented by a combination of thefirst layer and the second layer, specifically, a combination of thesecond organic compound, the third organic compound, and the fourthorganic compound, and as a result, an advantage that luminancedegradation with time can be suppressed is exhibited. However, thisadvantage is exhibited regardless of existence of the hole transportinglayer 112 and the electron transporting layer 113, and this can benaturally understood from the above explanation.

The emitted light is extracted outside through one or both of the firstelectrode 102 and the second electrode 104. Therefore, one or both ofthe first electrode 102 and the second electrode 104 have alight-transmitting property. When only the first electrode 102 has alight-transmitting property, the emitted light is extracted from thesubstrate side through the first electrode 102 as shown in FIG. 1A.Meanwhile, when only the second electrode 104 has a light-transmittingproperty, the emitted light is extracted from the side opposite to thesubstrate side through the second electrode 104 as shown in FIG. 1B.When each of the first electrode 102 and the second electrode 104 has alight-transmitting property, the emitted light is extracted from boththe substrate side and the side opposite to the substrate side throughthe first electrode 102 and the second electrode 104 as shown in FIG.1C.

The structure of the layers provided between the first electrode 102 andthe second electrode 104 is not limited to the aforementioned one. Astructure other than the aforementioned one may also be used as long asa light-emitting region in which holes and electrons are recombined isprovided in a portion apart from the first electrode 102 and the secondelectrode 104 so that quenching caused by the light-emitting region andmetal coming close to each other is suppressed, and moreover, as long asthe light-emitting layer includes the first layer 121 and the secondlayer 122.

That is to say, the stacked structure of the EL layer is notparticularly limited, and layers containing a substance with a highelectron transporting property, a substance with a high holetransporting property, a substance with a high electron injectingproperty, a substance with a high hole injecting property, a substancewith a bipolar property (a substance with a high electron and holetransporting property), and the like may be freely combined with thelight-emitting layer of the present invention.

In a structure of the light-emitting element shown in FIG. 2, over asubstrate 301, a second electrode 304 serving as a cathode, an EL layer303, and a first electrode 302 serving as an anode are sequentiallystacked. The EL layer 303 includes a hole transporting layer 312, alight-emitting layer 311, and an electron transporting layer 313. Thelight-emitting layer 311 has a first layer 321 and a second layer 322.The first layer 321 is provided closer to the side of the firstelectrode serving as an anode than to the second layer 322.

In this embodiment mode, the light-emitting element is manufactured overa substrate made of glass, plastic, or the like. When a plurality ofsuch light-emitting elements is manufactured over one substrate, apassive type light-emitting device can be manufactured. Moreover, forexample, a thin film transistor (TFT) may be formed over a substratemade of glass, plastic, or the like so that a light-emitting element ismanufactured over an electrode electrically connected to the TFT. Thus,an active matrix light-emitting device in which driving of thelight-emitting element is controlled by the TFT can be manufactured. Thestructure of the TFT is not particularly limited. The TFT may be eithera staggered type or an inverted staggered type. In addition, a drivercircuit formed over a TFT substrate may be formed using an N-type andP-type TFTs, or using either an N-type or P-type TFTs. Crystallinity ofa semiconductor film used for the TFT is not particularly limited,either. An amorphous semiconductor film may be used, or a crystallinesemiconductor film may be used.

In the light-emitting element of the present invention, a light-emittingregion is provided not at an interface between a light-emitting layerand a hole transporting layer or an interface between a light-emittinglayer and an electron transporting layer but in the vicinity of thecenter of the light-emitting layer, leading to no influence ofdeterioration of the hole transporting layer and the electrontransporting layer due to the light-emitting region coming close to thehole transporting layer or the electron transporting layer. Further,change in carrier balance with time (especially, change in the amount ofelectron injection with time) can be suppressed. Thus, a long-lifelight-emitting element that is not easily deteriorated can be obtained.Because the light-emitting layer of a light-emitting element of thepresent invention is formed using a compound that is stable even whenoxidation-reduction reaction is repeated, the light-emitting element isnot easily deteriorated even when light emission by recombination ofelectrons and holes is repeated, and thus a further long-lifelight-emitting element can be obtained.

Further, in the light-emitting element of the present invention,emission color of the first organic compound and emission color of thethird organic compound are similar colors. Accordingly, light emissionwith good color purity can be obtained even when not only the firstorganic compound but also the third organic compound emits light.

This embodiment mode can be arbitrarily combined with other embodimentmodes.

Embodiment Mode 2

In this embodiment mode, a mode of a light-emitting element in which aplurality of light-emitting units of the present invention are stacked(hereinafter this light-emitting element is referred to as astacked-type element) will be described with reference to FIG. 3. Thelight-emitting element is a stacked-type element including a pluralityof light-emitting units between a first electrode and a secondelectrode. Each of the light-emitting units may have a similar structureto that of the EL layer shown in Embodiment Mode 1. That is, thelight-emitting element shown in Embodiment Mode 1 is a light-emittingelement having one light-emitting unit, whereas the light-emittingelement described in this embodiment mode has a plurality oflight-emitting units.

In FIG. 3, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502, and a charge-generating layer 513 is provided between thefirst light-emitting unit 511 and the second light-emitting unit 512.The first electrode 501 and the second electrode 502 may be similar tothe electrodes shown in Embodiment Mode 1. The first light-emitting unit511 and the second light-emitting unit 512 may have either the samestructure or a different structure, which may be similar to thosedescribed in Embodiment Mode 1.

The charge-generating layer 513 contains a composite material of anorganic compound and a metal oxide. The composite material of an organiccompound and a metal oxide is the composite material shown in EmbodimentMode 1 and contains the organic compound and a metal oxide such asvanadium oxide, molybdenum oxide, or tungsten oxide. As the organiccompound, various compounds such as an aromatic amine compound,carbazole derivatives, aromatic hydrocarbon, and a high molecularcompound (oligomer, dendrimer, polymer, or the like) can be used. As theorganic compound, it is preferable to use the organic compound which hasa hole transporting property and has hole mobility of 10⁻⁶ cm²/Vs orhigher. However, other substances than these may also be used as long asthe hole transporting property is higher than the electron transportingproperty. The composite material of the organic compound and the metaloxide can achieve low-voltage driving and low-current driving because ofsuperior carrier injecting property and carrier transporting property.

Alternatively, the charge-generating layer 513 may be formed bycombining a layer containing the composite material of the organiccompound and the metal oxide with a layer formed using another material.For example, a layer containing the composite material of the organiccompound and the metal oxide may be combined with a layer containing acompound selected from substances with electron-donating properties anda compound with a high electron transporting property. Moreover, a layercontaining the composite material of the organic compound and the metaloxide may be combined with a transparent conductive film.

In any case, it is acceptable as long as the charge-generating layer 513interposed between the first light-emitting unit 511 and the secondlight-emitting unit 512 injects electrons into one of theselight-emitting units and holes to the other when voltage is applied tothe first electrode 501 and the second electrode 502. For example, inFIG. 3, it is acceptable as long as the charge-generating layer 513injects electrons into the first light-emitting unit 511 and holes tothe second light-emitting unit 512 in a case where a voltage is appliedso that a potential of the first electrode is higher than that of thesecond electrode.

This embodiment mode describes the light-emitting element having twolight-emitting units. However, the present invention can be similarlyapplied to a light-emitting element in which three or morelight-emitting units are stacked. When the charge-generating layer isprovided between the pair of electrodes so as to partition the pluralityof light-emitting units like the light-emitting element of thisembodiment mode, a long-life element in a high luminance region can berealized while keeping current density low. When the light-emittingelement is applied for lighting, voltage drop due to resistance of anelectrode material can be reduced, thereby achieving homogeneous lightemission in a large area. Moreover, a light-emitting device of low powerconsumption, which can be driven at low voltage, can be realized.

When light-emitting units have different emission colors, light emissionof desired color can be obtained as a whole light-emitting element. Forexample, in the light-emitting element having two light-emitting units,when emission color of the first light-emitting unit and emission colorof the third light-emitting unit are complementary colors, alight-emitting element emitting white light as a whole light-emittingelement can be obtained. It is to be noted that “complementary color”refers to a relation between colors which become achromatic color bybeing mixed. That is, white light emission can be obtained by mixture oflights obtained from substances emitting the lights of complementarycolors. Similarly, also in a light-emitting element including threelight-emitting units, white light emission can be similarly obtained asa whole light-emitting element in a case where emission color of a firstlight-emitting unit is red, emission color of a second light-emittingunit is green, and emission color of a third light-emitting unit isblue, for example.

This embodiment mode can be arbitrarily combined with other embodimentmodes.

Embodiment Mode 3

In this embodiment mode, a light-emitting device including alight-emitting element of the present invention will be described.

In this embodiment mode, a light-emitting device including alight-emitting element of the present invention in a pixel portion willbe described with reference to FIGS. 4A and 4B. FIG. 4A is a top viewshowing a light-emitting device, and FIG. 4B is a cross-sectional viewtaken along a line A-A′ and a line B-B′ in FIG. 4A. Reference numeral601 denotes a driver circuit portion (source driver circuit), 602denotes a pixel portion, and 603 denotes a driver circuit portion (gatedriver circuit), each of which is shown by a dotted line. Referencenumeral 604 denotes a sealing substrate, 605 denotes a sealing member,and 607 denotes a space surrounded by the sealing member 605.

A lead wiring 608 is a wiring for transmitting signals to be input tothe source driver circuit 601 and the gate driver circuit 603, andreceives a video signal, a clock signal, a start signal, a reset signal,and the like from an FPC (Flexible Printed Circuit) 609 that is anexternal input terminal. Although only the FPC is shown here, a printedwiring board (PWB) may be attached to the PPC. The light-emitting devicein this specification includes not only the light-emitting deviceitself, but also a state where the FPC or the PWB is attached to thelight-emitting device.

Next, a cross-sectional structure will be described with reference toFIG. 4B. Although the driver circuit portions and the pixel portion areformed over an element substrate 610, FIG. 4B shows the source drivercircuit 601 that is the driver circuit portion and one pixel in thepixel portion 602.

The source driver circuit 601 includes a CMOS circuit formed bycombining an N-channel TFT 623 and a P-channel TFT 624. Alternatively,the driver circuit may be formed using a CMOS circuit, a PMOS circuit,or an NMOS circuit. In this embodiment mode, the integrated drivercircuit that is formed over the substrate is shown; however, the drivercircuit is not necessarily formed over the substrate and may be formedoutside the substrate.

The pixel portion 602 includes a plurality of pixels each having aswitching TFT 611, a current controlling TFT 612, and a first electrode613 that is electrically connected to a drain of the current controllingTFT 612. An insulator 614 is formed to cover an edge portion of thefirst electrode 613. Here, the insulator 614 is formed using a positivephotosensitive acrylic resin film.

In order to improve coverage, an upper edge portion or a lower edgeportion of the insulator 614 is formed so as to have a curved surfacewith curvature. For example, if positive photosensitive acrylic is usedfor the insulator 614, it is preferable that only the upper edge portionof the insulator 614 have a curved surface with a radius of curvature of0.2 to 3 μm. Either a negative type which becomes insoluble in anetchant by light irradiation or a positive type which becomes soluble inan etchant by light irradiation can be used as the insulator 614.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. As a material used for the first electrode 613, variousmetals, alloys, electroconductive compounds, or a mixture thereof can beused. If the first electrode is used as an anode, it is preferable touse, among those materials, a metal, an alloy, an electroconductivecompound, a mixture thereof, or the like with a high work function (workfunction of 4.0 eV or higher). For example, it is possible to use asingle layer film of indium oxide-tin oxide film containing silicon,indium oxide-zinc oxide film, a titanium nitride film, a chromium film,a tungsten film, a zinc film, a platinum film, or the like. It is alsopossible to use a stacked film such as a stack of a film containingtitanium nitride and a film mainly containing aluminum or a three-layerstructure of a titanium nitride film, a film mainly containing aluminum,and a titanium nitride film. With the stacked structure, a low wiringresistance, favorable ohmic contact, and a function as an anode can beachieved.

The EL layer 616 is formed by various methods such as an evaporationmethod using an evaporation mask, an ink-jet method, and a spin coatmethod. The EL layer 616 includes the light-emitting layer shown inEmbodiment Modes 1 and 2. As another material included in the EL layer616, a low molecular compound or a high molecular compound (includingoligomer or dendrimer) may be used. As the material for the EL layer,not only an organic compound but also an inorganic compound may be used.

As a material used for the second electrode 617, various metals, alloys,electroconductive compounds, or a mixture thereof can be used. If thesecond electrode is used as a cathode, it is preferable to use, amongthose materials, a metal, an alloy, an electroconductive compound, amixture thereof, or the like with a low work function (a work functionof 3.8 eV or lower). For example, an element that belongs to Group 1 or2 of the periodic table, that is, an alkali metal such as lithium (Li)or cesium (Cs), an alkaline earth metal such as magnesium (Mg), calcium(Ca), or strontium (Sr), an alloy containing these (magnesium-silver,aluminum-lithium), or the like can be given. If light generated in theEL layer 616 is transmitted through the second electrode 617, the secondelectrode 617 can be formed using a stack of a metal thin film and atransparent conductive film (indium oxide-tin oxide (ITO), indiumoxide-tin oxide containing silicon or silicon oxide, indium oxide-zincoxide (IZO), indium oxide containing tungsten oxide and zinc oxide(IWZO), or the like).

When the sealing substrate 604 and the element substrate 610 areattached to each other with the sealing member 605, a light-emittingelement 618 is provided in the space 607 surrounded by the elementsubstrate 610, the sealing substrate 604, and the sealing member 605.The space 607 may be filled with filler, and may be filled with an inertgas (such as nitrogen and argon), the sealing member 605, or the like.

An epoxy-based resin is preferably used for the sealing member 605. Thematerial preferably allows as little moisture and oxygen as possible topenetrate. As a material for the sealing substrate 604, a plasticsubstrate made of FRP (Fiberglass-Reinforced Plastics), PVF (polyvinylfluoride), polyester, acrylic, or the like can be used besides a glasssubstrate or a quartz substrate.

In this manner, the light-emitting device including the light-emittingelement of the present invention can be obtained.

The light-emitting device of the present invention has thelight-emitting element described in Embodiment Modes 1 and 2.Accordingly, by the long-life light-emitting element of the presentinvention being included, a long-life light-emitting device can beobtained. Further, a light-emitting device which is superior in colorpurity can be obtained.

As described above, in this embodiment mode, an active matrix typelight-emitting device in which operation of a light-emitting element iscontrolled by a transistor is described. Alternatively, a passive matrixtype light-emitting device may also be used. FIG. 5 shows a perspectiveview of a passive matrix type light-emitting device which ismanufactured by application of the present invention. In FIG. 5, an ELlayer 955 is provided between an electrode 952 and an electrode 956 overa substrate 951. An edge portion of the electrode 952 is covered with aninsulating layer 953. Further, a partition layer 954 is provided overthe insulating layer 953. A side wall of the partition layer 954 slopesso that a distance between one side wall and the other side wall becomesnarrow toward a substrate surface. In other words, a cross section ofthe partition layer 954 in the direction of a short side is trapezoidal,and a base (a side facing in the same direction as a plane direction ofthe insulating layer 953 and in contact with the insulating layer 953)is shorter than an upper side (a side facing in the same direction asthe plane direction of the insulating layer 953 and not in contact withthe insulating layer 953). By the partition layer 954 being provided inthis manner, defects of the light-emitting element due to staticelectricity or the like can be prevented. Also in the passive matrixtype light-emitting device, by the long-life light-emitting element ofthe present invention being included, a long-life light-emitting devicecan be obtained. Further, a light-emitting device which is superior incolor purity can be obtained.

Embodiment Mode 4

In this embodiment mode, an electronic device of the present inventionincluding the light-emitting device described in Embodiment Mode 3 as apart thereof will be described. That is, the electronic device of thepresent invention includes a long-life display portion having thelight-emitting element shown in Embodiment Modes 1 and 2. Further, sincethe light-emitting element which is superior in color purity isincluded, a display portion which is superior in color reproductivitycan be obtained.

Examples of the electronic device manufactured by using thelight-emitting device of the present invention are as follows: a camerasuch as a video camera or a digital camera, a goggle type display, anavigation system, a sound reproducing device (a car audio system, anaudio component, or the like), a computer, a game machine, a portableinformation terminal (a mobile computer, a mobile phone, a mobile gamemachine, an electronic book, or the like), an image reproducing deviceprovided with a recording medium (specifically, a device for reproducinga recording medium such as a digital versatile disc (DVD) and having adisplay device for displaying the image), and the like. FIGS. 6A to 6Dshow specific examples of these electronic devices.

FIG. 6A shows a television device according to the present invention,which includes a chassis 9101, a support base 9102, a display portion9103, a speaker portion 9104, a video input terminal 9105, and the like.In this television device, the display portion 9103 includeslight-emitting elements similar to those described in Embodiment Modes 1and 2, which are arranged in a matrix. The light-emitting element has afeature that the life is long. Since the display portion 9103 includingthe light-emitting element also has the similar feature, this televisiondevice has a feature that the life is long. That is, a television devicethat can withstand long-time use can be provided. Further, since alight-emitting element which is superior in color purity is included, atelevision device having a display portion which is superior in colorreproductivity can be obtained.

FIG. 6B shows a computer according to the present invention, whichincludes a main body 9201, a chassis 9202, a display portion 9203, akeyboard 9204, an external connection port 9205, a pointing device 9206,and the like. In this computer, the display portion 9203 includeslight-emitting elements similar to those described in Embodiment Modes 1and 2, which are arranged in a matrix. The light-emitting element has afeature that the life is long. Because the display portion 9203including the light-emitting element also has the similar feature, thiscomputer has a feature that the life is long. That is, a computer thatcan withstand long-time use can be provided. Further, because alight-emitting element which is superior in color purity is included, acomputer having a display device which is superior in colorreproductivity can be obtained.

FIG. 6C shows a mobile phone according to the present invention, whichincludes a main body 9401, a chassis 9402, a display portion 9403, anaudio input portion 9404, an audio output portion 9405, an operation key9406, an external connection port 9407, an antenna 9408, and the like.In this mobile phone, the display portion 9403 includes light-emittingelements similar to those described in Embodiment Modes 1 and 2, whichare arranged in a matrix. The light-emitting element has a feature thatthe life is long. Because the display portion 9403 including thelight-emitting element also has the similar feature, this mobile phonehas a feature that the life is long. That is, a mobile phone that canwithstand long-time use can be provided. Further, because alight-emitting element which is superior in color purity is included, amobile phone having a display device which is superior in colorreproductivity can be obtained.

FIG. 6D shows a camera according to the present invention, whichincludes a main body 9501, a display portion 9502, a chassis 9503, anexternal connection port 9504, a remote control receiving portion 9505,an image receiving portion 9506, a battery 9507, an audio input portion9508, an operation key 9509, an eye-piece portion 9510, and the like. Inthis camera, the display portion 9502 includes light-emitting elementssimilar to those described in Embodiment Modes 1 and 2, which arearranged in a matrix. The light-emitting element has a feature that thelife is long. Because the display portion 9502 including thelight-emitting element also has the similar feature, this camera has afeature that the life is long. That is, a camera that can withstandlong-time use can be provided. Further, because a light-emitting elementwhich is superior in color purity is included, a camera having a displaydevice which is superior in color reproductivity can be obtained.

As described above, the applicable range of the light-emitting device ofthe present invention is so wide that the light-emitting device can beapplied to electronic devices in various fields. By using thelight-emitting device of the present invention, an electronic devicehaving a long-life display portion that can withstand long-time use canbe provided. Further, an electronic device having a display portionwhich is superior in color reproductivity can be obtained.

Alternatively, the light-emitting device of the present invention canalso be used as a lighting device. One mode using the light-emittingelement of the present invention as a lighting device will be describedwith reference to FIG. 7.

FIG. 7 shows an example of a liquid crystal display device using thelight-emitting device of the present invention as a backlight. Theliquid crystal display device shown in FIG. 7 includes a chassis 901, aliquid crystal layer 902, a backlight 903, and a chassis 904, and theliquid crystal layer 902 is connected to a driver IC 905. Thelight-emitting device of the present invention is used for the backlight903, and current is supplied through a terminal 906.

By using the light-emitting device of the present invention as thebacklight of the liquid crystal display device, a long-life backlightcan be obtained. The light-emitting device of the present invention is alighting device with planar light emission, and can have a large area.Therefore, the backlight can have a large area, and a liquid crystaldisplay device having a large area can be obtained. Furthermore, thelight-emitting device of the present invention is thin and consumes lowpower; therefore, a thin shape and low power consumption of a displaydevice can also be achieved.

FIG. 8 shows an example of using the light-emitting device, to which thepresent invention is applied, as a table lamp that is a lighting device.A table lamp shown in FIG. 8 has a chassis 2001 and a light source 2002,and the light-emitting device of the present invention is used as thelight source 2002. Because the life of the light-emitting device of thepresent invention is long, the life of the table lamp is also long.

FIG. 9 shows an example of using the light-emitting device, to which thepresent invention is applied, as an indoor lighting device 3001. Sincethe light-emitting device of the present invention can have a largerarea, the light-emitting device of the present invention can be used asa lighting device having a large area. Further, because the life of thelight-emitting device of the present invention is long, thelight-emitting device of the present invention can be used as along-life lighting device. A television device 3002 according to thepresent invention as described in FIG. 6A is placed in a room in whichthe light-emitting device, to which the present invention is applied, isused as the indoor lighting device 3001. Thus, public broadcasting andmovies can be watched. In such a case, since each of the devices has thelong life, the number of replacing the lighting device and thetelevision device can be reduced, whereby the environmental load can bereduced.

Embodiment 1

In this embodiment, a light-emitting element of the present inventionwill be specifically described with reference to FIG. 10. A structuralformula of an organic compound used in this embodiment is shown below.

(Light-Emitting Element 1)

First, indium oxide-tin oxide containing silicon oxide was depositedover a glass substrate 2101 by a sputtering method, leading to theformation of a first electrode 2102. It is to be noted that a thicknessthereof was 110 nm and an electrode area was 2 mm×2 mm.

Next, the substrate, over which the first electrode 2102 was formed, wasfixed to a substrate holder in a vacuum evaporation apparatus so thatthe side, on which the first electrode 2102 was formed, faced downward.Subsequently, after the pressure of the vacuum evaporation apparatus wasreduced to approximately 10⁻⁴ Pa, a layer 2103 containing a compositematerial was formed on the first electrode 2102 by co-evaporation of4,4′-bis[N-(1-napthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum oxide (VI). A thickness thereof was adjusted to be 50 nm anda weight ratio of NPB to molybdenum oxide (VI) was adjusted to be 4:1NPB:molybdenum oxide). It is to be noted that the co-evaporation methodis an evaporation method in which evaporation is performedsimultaneously using a plurality of evaporation sources in oneevaporation chamber.

Subsequently, 4,4′-bis[N-(1-napthyl)-N-phenylamino]biphenyl(abbreviation: NPB) was deposited to a thickness of 10 nm by anevaporation method using resistance heating, leading to the formation ofa hole transporting layer 2104.

Furthermore, a light-emitting layer 2105 was formed on the holetransporting layer 2104. First, on the hole transporting layer 2104, afirst light-emitting layer 2121 was formed to a thickness of 30 nm byco-evaporation of 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA) that is a second organic compound withN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazole-3-amine(abbreviation: 2PCAPA) that is a first organic compound. Here, a weightratio of CzPA to 2PCAPA was adjusted to be 1:0.05 (=CzPA:2PCAPA).Further, on the first light-emitting layer 2121, a second light-emittinglayer 2122 was formed to a thickness of 10 nm by co-evaporation oftris(8-quinolinolato)aluminum(III) (abbreviation: Alq) that is a fourthorganic compound with N,N′-diphenylquinacridone (abbreviation: DPQd)that is a third organic compound. Here, a weight ratio of Alq to DPQdwas adjusted to be 1:0.005 (=Alq:DPQd).

Subsequently, bathophenanthroline (abbreviation: BPhen) was deposited toa thickness of 30 nm on the light-emitting layer 2105 by an evaporationmethod using resistance heating, leading to the formation of an electrontransporting layer 2106, and thereafter lithium fluoride (LiF) wasdeposited to a thickness of 1 nm on the electron transporting layer2106, leading to the formation of an electron injecting layer 2107.

Finally, aluminum was deposited to a thickness of 200 nm by anevaporation method using resistance heating, leading to the formation ofa second electrode 2108. Accordingly, a light-emitting element 1 wasmanufactured.

(Comparative Light-Emitting Element 2)

First, indium oxide-tin oxide containing silicon oxide was depositedover a glass substrate by a sputtering method, leading to the formationof a first electrode. It is to be noted that a thickness thereof was 110nm and an electrode area was 2 mm×2 mm.

Next, the substrate, over which the first electrode was formed, wasfixed to a substrate holder in a vacuum evaporation apparatus so thatthe side, on which the first electrode was formed, faced downward.Subsequently, after the pressure of the vacuum evaporation apparatus wasreduced to approximately 10⁻⁴ Pa, a layer containing a compositematerial was formed on the first electrode by co-evaporation of4,4′-bis[N-(1-napthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum oxide (VI). A thickness thereof was adjusted to be 50 nm anda weight ratio of NPB to molybdenum oxide (VI) was adjusted to be 4:1(=NPB:molybdenum oxide).

Subsequently, 4,4′-bis[N-(1-napthyl)-N-phenylamino]biphenyl(abbreviation: NPB) was deposited to a thickness of 10 nm by anevaporation method using resistance heating, leading to the formation ofa hole transporting layer.

Furthermore, a light-emitting layer was formed on the hole transportinglayer. The light-emitting layer was formed to a thickness of 40 nm byco-evaporation of 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA) withN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazole-3-amine(abbreviation: 2PCAPA). Here, a weight ratio of CzPA to 2PCAPA wasadjusted to be 1:0.05 (=CzPA:2PCAPA).

Thereafter, bathophenanthroline (abbreviation: BPhen) was deposited to athickness of 30 nm on the light-emitting layer by an evaporation methodusing resistance heating, leading to the formation of an electrontransporting layer.

Lithium fluoride (LiF) was deposited to a thickness of 1 nm on theelectron transporting layer, leading to the formation of an electroninjecting layer.

Finally, aluminum was deposited to a thickness of 200 nm by anevaporation method using resistance heating, leading to the formation ofa second electrode. Accordingly, a comparative light-emitting element 2was manufactured.

FIG. 11 shows a current density-luminance characteristic of thelight-emitting element 1. FIG. 12 shows a voltage-luminancecharacteristic. FIG. 13 shows a luminance-current efficiencycharacteristic. FIG. 14 shows an emission spectrum when a current of 1mA flows to the light-emitting element 1.

In the light-emitting element 1, the CIE chromaticity coordinates atluminance of 3000 cd/m² were (x, y)=(0.29, 0.62), and emission of greenlight was obtained. Current efficiency at luminance of 3000 cd/m² was10.7 cd/A; voltage, 5.8 V; and current density, 29.4 mA/cm².

A continuous lighting test was performed on the light-emitting element 1by constant current driving at an initial luminance of 3000 cd/m².Through the test, it was found that the light-emitting element 1 is along-life light-emitting element with luminance of 89% of the initialluminance even after 640 hours have passed. On the other hand, acontinuous lighting test was similarly performed on the comparativelight-emitting element 2 at an initial luminance of 3000 cd/m². Throughthe test, it was found that the comparative light-emitting element 2 hasluminance of 76% of the initial luminance after 640 hours have passed,which was the shorter life than that of the light-emitting element 1.

Accordingly, it was revealed that a long-life light-emitting element canbe obtained by application of the present invention.

Embodiment 2

In this embodiment, a light-emitting element of the present inventionwill be specifically described with reference to FIG. 10.

(Light-Emitting Element 3)

First, indium oxide-tin oxide containing silicon oxide was depositedover a glass substrate 2101 by a sputtering method, leading to theformation of a first electrode 2102. It is to be noted that a thicknessthereof was 110 nm and an electrode area was 2 mm×2 mm.

Next, the substrate, over which the first electrode 2102 was formed, wasfixed to a substrate holder in a vacuum evaporation apparatus so thatthe side, on which the first electrode 2102 was formed, faced downward.Subsequently, after the pressure of the vacuum evaporation apparatus wasreduced to approximately 10⁻⁴ Pa, a layer 2103 containing a compositematerial was formed on the first electrode 2102 by co-evaporation of4,4′-bis[N-(1-napthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum oxide (VI). A thickness thereof was adjusted to be 50 nm anda weight ratio of NPB to molybdenum oxide (VI) was adjusted to be 4:1(=NPB:molybdenum oxide).

Next, 4,4′-bis[N-(1-napthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was deposited to a thickness of 10 nm by an evaporation method usingresistance heating, leading to the formation of a hole transportinglayer 2104.

Furthermore, a light-emitting layer 2105 was formed on the holetransporting layer 2104. First, on the hole transporting layer 2104, afirst light-emitting layer 2121 was formed to a thickness of 30 nm byco-evaporation of 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA) that is a second organic compound withN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazole-3-amine(abbreviation: 2PCAPA) that is a first organic compound. Here, a weightratio of CzPA to 2PCAPA was adjusted to be 1:0.05 (=CzPA:2PCAPA).Further, on the first light-emitting layer 2121, a second light-emittinglayer 2122 was formed to a thickness of 10 nm by co-evaporation oftris(8-quinolinolato)aluminum(III) (abbreviation: Alq) that is a fourthorganic compound with N,N′-diphenylquinacridone (abbreviation: DPQd)that is a third organic compound. Here, a weight ratio of Alq to DPQdwas adjusted to be 1:0.05 (=Alq:DPQd).

Thereafter, Alq was deposited to a thickness of 30 nm on thelight-emitting layer 2105 by an evaporation method using resistanceheating, leading to the formation of an electron transporting layer2106.

Further, lithium fluoride (LiF) was deposited to a thickness of 1 nm onthe electron transporting layer 2106, leading to the formation of anelectron injecting layer 2107.

Finally, aluminum was deposited to a thickness of 200 nm by anevaporation method using resistance heating, leading to the formation ofa second electrode 2108. Accordingly, a light-emitting element 3 wasmanufactured.

(Comparative Light-Emitting Element 4)

First, indium oxide-tin oxide containing silicon oxide was depositedover a glass substrate by a sputtering method, leading to the formationof a first electrode. It is to be noted that a thickness thereof was 110nm and an electrode area was 2 mm×2 mm.

Subsequently, the substrate, over which the first electrode was formed,was fixed to a substrate holder in a vacuum evaporation apparatus sothat the side, on which the first electrode was formed, faced downward.Subsequently, after the pressure of the vacuum evaporation apparatus wasreduced to approximately 10⁻⁴ Pa, a layer containing a compositematerial was formed on the first electrode by co-evaporation of4,4′-bis[N-(1-napthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum oxide (VI). A thickness thereof was adjusted to be 50 nm anda weight ratio of NPB to molybdenum oxide (VI) was adjusted to be 4:1(=NPB:molybdenum oxide).

Next, 4,4′-bis[N-(1-napthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was deposited to a thickness of 10 nm by an evaporation method usingresistance heating, leading to the formation of a hole transportinglayer.

Thereafter, a light-emitting layer was formed on the hole transportinglayer. The light-emitting layer was formed to a thickness of 40 nm byco-evaporation of 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA) withN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazole-3-amine(abbreviation: 2PCAPA). Here, a weight ratio of CzPA to 2PCAPA wasadjusted to be 1:0.05 (=CzPA:2PCAPA).

Thereafter, Alq was deposited to a thickness of 30 nm on thelight-emitting layer by an evaporation method using resistance heating,leading to the formation of an electron transporting layer.

Further, lithium fluoride (LiF) was deposited to a thickness of 1 nm onthe electron transporting layer, leading to the formation of an electroninjecting layer.

Finally, aluminum was deposited to a thickness of 200 nm by anevaporation method using resistance heating, leading to the formation ofa second electrode. Accordingly, a comparative light-emitting element 4was manufactured.

FIG. 15 shows a current density-luminance characteristic of thelight-emitting element 3. FIG. 16 shows a voltage-luminancecharacteristic. FIG. 17 shows a luminance-current efficiencycharacteristic. FIG. 18 shows an emission spectrum when a current of 1mA flows to the light-emitting element 3.

In the light-emitting element 3, the CIE chromaticity coordinates atluminance of 3000 cd/m² were (x, y)=(0.29, 0.62), and emission of greenlight was obtained. Current efficiency at luminance of 3000 cd/m² was11.0 cd/A; voltage, 8.0 V; and current density, 28.3 mA/cm².

A continuous lighting test was performed on the light-emitting element 3by constant current driving at an initial luminance of 3000 cd/m².Through the test, it was found that the light-emitting element 3 is along-life light-emitting element with luminance of 90% of the initialluminance even after 640 hours have passed. On the other hand, acontinuous lighting test was similarly performed on the comparativelight-emitting element 4 at an initial luminance of 3000 cd/m². Throughthe test, it was found that the comparative light-emitting element 4 hasluminance of 88% of the initial luminance after 470 hours have passed,which was the shorter life than that of the light-emitting element 3.

Accordingly, it was revealed that a long-life light-emitting element canbe obtained by application of the present invention.

Embodiment 3

In this embodiment, a reduction reaction characteristic oftris(8-quinolinolato)aluminum(III) (abbreviation: Alq) andN,N′-diphenylquinacridone (abbreviation: DPQd), which were used for thesecond layer in each of the light-emitting element 1 and thelight-emitting element 3 manufactured in Embodiment 1 and Embodiment 2,was examined by cyclic voltammetry (CV) measurement. Further, LUMOlevels of Alq and DPQd were obtained from the measurement. Themeasurement was performed using an electrochemical analyzer (ALS model600A, manufactured by BAS Inc.).

As for a solution used in the CV measurement, dehydrateddimethylformamide (DMF, manufactured by Aldrich, 99.8%, catalog number:22705-6) was used as a solvent. Tetra-n-butylammonium perchlorate(n-Bu₄NClO₄, manufactured by Tokyo Chemical Industry Co., Ltd., catalognumber: T0836), which was a supporting electrolyte, was dissolved in thesolvent such that the concentration of the tetra-n-butylammoniumperchlorate was 100 mmol/L. Moreover, the object to be measured wasdissolved such that the concentration thereof was set to be 1 mmol/L.Further, a platinum electrode (a PIE platinum electrode, manufactured byBAS Inc.) was used as a work electrode. A platinum electrode (a VC-3 Ptcounter electrode (5 cm), manufactured by BAS Inc.) was used as anauxiliary electrode. An Ag/Ag⁺ electrode (an RE5 nonaqueous solventreference electrode, manufactured by BAS Inc.) was used as a referenceelectrode. It is to be noted that the measurement was conducted at roomtemperature (20 to 25° C.).

(Calculation of Potential Energy of the Reference Electrode with Respectto a Vacuum Level)

First, potential energy (eV) of the reference electrode (Ag/Ag⁺electrode) used in Embodiment 3 with respect to a vacuum level wascalculated. That is, a Fermi level of the Ag/Ag⁺ electrode wascalculated. It is known that an oxidation-reduction potential offerrocene in methanol is +0.610 [V vs. SHE] with respect to a standardhydrogen electrode (Reference: Christian R. Goldsmith et al., J. Am.Chem. Soc., Vol. 124, No. 1, pp. 83-96, 2002). On the other hand, anoxidation-reduction potential of ferrocene in methanol was obtainedusing the reference electrode used in Embodiment 3 was +0.20 V [vs.Ag/Ag⁺]. Accordingly, it was found that potential energy of thereference electrode used in Embodiment 3 was lowered by 0.41 [eV] withrespect to the standard hydrogen electrode.

Also, it is known that potential energy of the standard hydrogenelectrode from a vacuum level is −4.44 eV (Reference: Toshihiro Ohnishiand Tamami Koyama, High molecular EL material, Kyoritsu shuppan, pp.64-67). From the above, potential energy of the reference electrode usedin Embodiment 3 with respect to a vacuum level was calculated such that−4.44−0.41=−4.85 [eV].

(Measurement Example 1; Alq)

In Measurement Example 1, a reduction reaction characteristic of Alq wasmeasured by cyclic voltammetry (CV) measurement. The scan speed was setto 0.1 V/sec. FIG. 19 shows a result of the measurement. In themeasurement of the reduction reaction characteristic, a potential of aworking electrode with respect to a reference electrode was changed from−0.69 V to −2.40 V, and then changed from −2.40 V to −0.69 V.

As is seen from FIG. 19, a reduction peak potential E_(pc) is −2.20 Vand an oxidation peak potential E_(pa) is −2.12 V. Accordingly, awave-half potential (an intermediate potential between E_(pc) andE_(pa)) can be calculated to be −2.16 V. This shows that Alq is reducedby electrical energy of −2.16 [V vs. Ag/Ag⁺], and this energycorresponds to a LUMO level. Here, as described above, potential energyof the reference electrode used in Embodiment 3 with respect to a vacuumlevel is −4.85 [eV]. Accordingly, it was found that a LUMO level of Alqwas such that −4.85−(−2.16)=−2.69 [eV].

(Measurement Example 2; DPQd)

In Measurement Example 2, a reduction reaction characteristic of DPQdwas measured by cyclic voltammetry (CV) measurement. The scan speed wasset to 0.1 V/sec. FIG. 20 shows a result of the measurement. In themeasurement of the reduction reaction characteristic, a potential of aworking electrode with respect to a reference electrode was changed from−0.40 V to −2.10 V, and then changed from −2.10 V to −0.40 V. BecauseDPQd has poor solubility, an undissolved residue of DPQd was generatedin spite of preparation trial in which a solution is made to have theconcentration of 1 mmol/L. Accordingly, in a state where the undissolvedresidue was precipitated, a supernatant solution was extracted and usedfor the measurement.

As is seen from FIG. 20, a reduction peak potential E_(pc) is −1.69 Vand an oxidation peak potential E_(pa) is −1.63 V. Accordingly, awave-half potential (an intermediate potential between E_(pc) andE_(pa)) can be calculated to be −1.66 V. This shows that DPQd ischemically reduced by electrical energy of −1.66 [V vs. Ag/Ag⁺], andthis energy corresponds to a LUMO level. Here, as described above,potential energy of the reference electrode used in Embodiment 3 withrespect to a vacuum level is −4.85 [eV]. Accordingly, it was found thata LUMO level of DPQd was such that −4.85−(−1.66)=−3.19 [eV].

When LUMO levels of Alq and DPQd obtained as described above arecompared with each other, it is found that the LUMO level of DPQd islower than that of Alq by 0.50 [eV]. This means that, by addition ofDPQd to Alq, DPQd acts as an electron trap. Accordingly, the elementstructure of Embodiment 1 and Embodiment 2 is a preferred structure forthe present invention, in which DPQd is used as the third organiccompound and Alq is used as the fourth organic compound in the secondlayer of the light-emitting element of the present invention.

This application is based on Japanese Patent Application serial No.2006-155159 filed in Japan Patent Office on Jun. 2, 2006, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light emitting device comprising: an anode; afirst layer over the anode, the first layer being configured to emitlight; a second layer over the first layer; an electron transportinglayer over the second layer; and a cathode over the electrontransporting layer, wherein the second layer is capable of retardingtransfer of electron injected to the second layer from the cathodethrough the electron transporting layer, and wherein the electrontransporting layer is between the second layer and the cathode.
 2. Thelight emitting device according to claim 1, wherein an electrontransporting property of the first layer is greater than a holetransporting property.
 3. The light emitting device according to claim1, wherein the first layer comprises a first compound dispersed in asecond compound, and wherein the second compound has an electrontransporting property greater than a hole transporting property.
 4. Thelight emitting device according to claim 1, wherein the second layercomprises a third compound dispersed in a fourth compound, and whereinthe fourth compound has an electron transporting property greater than ahole transporting property.
 5. The light emitting device according toclaim 1, wherein the second layer comprises a third compound dispersedin a fourth compound, and wherein the retardation of the transfer ofelectron is achieved by an electron-trapping ability of the thirdcompound in the fourth compound.
 6. The light emitting device accordingto claim 1, wherein the second layer is in contact with the first layer.7. A lighting device comprising the light emitting device according toclaim
 1. 8. A table lamp comprising the light emitting device accordingto claim
 1. 9. A light emitting device comprising: an anode; a holetransporting layer over the anode; a first layer over the holetransporting layer, the first layer being configured to emit light; asecond layer over the first layer; an electron transporting layer overthe second layer; and a cathode over the electron transporting layer,wherein the second layer is capable of preventing electron, which isinjected from the cathode through the electron transporting layer, fromreaching the hole transporting layer, and wherein the electrontransporting layer is between the second layer and the cathode.
 10. Thelight emitting device according to claim 9, wherein an electrontransporting property of the first layer is greater than a holetransporting property.
 11. The light emitting device according to claim9, wherein the first layer comprises a first compound dispersed in asecond compound, and wherein the second compound has an electrontransporting property greater than a hole transporting property.
 12. Thelight emitting device according to claim 9, wherein the second layercomprises a third compound dispersed in a fourth compound, and whereinthe fourth compound has an electron transporting property greater than ahole transporting property.
 13. The light emitting device according toclaim 9, wherein the second layer comprises a third compound dispersedin a fourth compound, and wherein the prevention of electron fromreaching the hole transporting layer is achieved by an electron-trappingability of the third compound in the fourth compound.
 14. The lightemitting device according to claim 9, wherein the second layer is incontact with the first layer.
 15. A lighting device comprising the lightemitting device according to claim
 9. 16. A table lamp comprising thelight emitting device according to claim
 9. 17. A light emitting devicecomprising: an anode; a first layer over the anode, the first layerbeing configured to emit light; a second layer over the first layer; anelectron transporting layer over the second layer; and a cathode overthe electron transporting layer, wherein the second layer is capable oftrapping electron, which is injected from the cathode through theelectron transporting layer, and wherein the electron transporting layeris between the second layer and the cathode.
 18. The light emittingdevice according to claim 17, wherein an electron transporting propertyof the first layer is greater than a hole transporting property.
 19. Thelight emitting device according to claim 17, wherein the first layercomprises a first compound dispersed in a second compound, and whereinthe second compound has an electron transporting property greater than ahole transporting property.
 20. The light emitting device according toclaim 17, wherein the second layer comprises a third compound dispersedin a fourth compound, and wherein the fourth compound has an electrontransporting property greater than a hole transporting property.
 21. Thelight emitting device according to claim 17, wherein the second layercomprises a third compound dispersed in a fourth compound, and whereinthe trapping of electron in the first layer is achieved by an energydifference between a LUMO of the third compound and a LUMO of the fourthcompound, which is equal to or larger than 0.3 eV.
 22. The lightemitting device according to claim 17, wherein the second layer is incontact with the first layer.
 23. A lighting device comprising the lightemitting device according to claim
 17. 24. A table lamp comprising thelight emitting device according to claim
 17. 25. A light emitting devicecomprising: a first electrode; a first light emitting unit over thefirst electrode; a charge generation layer over the first light emittingunit; a second light emitting unit over the charge generation layer; anda second electrode over the second light emitting unit, wherein thecharge generation layer is capable of injecting a hole to the secondlight emitting unit and an electron to the first light emitting unit,wherein each of the first light emitting unit and the second lightemitting unit comprises: a first layer comprising a first compound and asecond compound; and a second layer over the first layer, the secondlayer comprising a third compound and a fourth compound, wherein thefirst compound is dispersed in the second compound which has an electrontransporting property greater than a hole transporting property, whereinthe third compound is dispersed in the fourth compound and has anelectron trapping ability in the second layer, and wherein the fourthcompound has an electron transporting property greater than a holetransporting property.
 26. The light emitting device according to claim25, wherein the charge generation layer comprises an organic compoundand a metal oxide.
 27. The light emitting device according to claim 26,wherein the metal oxide is molybdenum oxide.
 28. The light emittingdevice according to claim 26, wherein the organic compound has a holetransporting property greater than an electron transporting property.29. The light emitting device according to claim 25, wherein a LUMOlevel of the third compound is lower than a LUMO level of the fourthcompound by 0.3 eV or more.
 30. A lighting device comprising the lightemitting device according to claim
 25. 31. A table lamp comprising thelight emitting device according to claim
 25. 32. A light emitting devicecomprising: a first electrode; a first light emitting unit over thefirst electrode; a charge generation layer over the first light emittingunit; a second light emitting unit over the charge generation layer; anda second electrode over the second light emitting unit, wherein anemission color of the first light emitting unit and an emission color ofthe second light emitting unit are complementary to each other, whereinthe charge generation layer is capable of injecting a hole to the secondlight emitting unit and an electron to the first light emitting unit,wherein the second light emitting unit comprises: a first layer over thecharge generation layer, the first layer comprising a first compound anda second compound; and a second layer over the first layer, the secondlayer comprising a third compound and a fourth compound, wherein thefirst compound is dispersed in the second compound which has an electrontransporting property greater than a hole transporting property, whereinthe third compound is dispersed in the fourth compound and has anelectron trapping ability in the second layer, and wherein the fourthcompound has an electron transporting property greater than a holetransporting property.
 33. The light emitting device according to claim32, wherein the charge generation layer comprises an organic compoundand a metal oxide.
 34. The light emitting device according to claim 33,wherein the metal oxide is molybdenum oxide.
 35. The light emittingdevice according to claim 33, wherein the organic compound has a holetransporting property greater than an electron transporting property.36. The light emitting device according to claim 32, wherein a LUMOlevel of the third compound is lower than a LUMO level of the fourthcompound by 0.3 eV or more.
 37. The light emitting device according toclaim 32, wherein the first compound is capable of emitting green light.38. The light emitting device according to claim 32, wherein the thirdcompound is capable of emitting red light.
 39. A lighting devicecomprising the light emitting device according to claim
 32. 40. A tablelamp comprising the light emitting device according to claim 32.